Add Stratum support
Ignore configure.lineno and config.status.lineno
Replace sscanf() with strtol() in hex2bin()
Perform global libcurl initialization

curl_easy_init() is not thread-safe unless curl_global_init() has
already been called.
It seems that SSL initialization is slow on Windows, so only do that
if the mining server has to be accessed via HTTPS.
Thanks to @martinwguy for pointing this out.